This invention relates to a method for treating titanium dioxide pigment, a titanium dioxide pigment resulting therefrom, and electrophoretic coating compositions which incorporate such pigment.
Electrophoretic deposition of coating compositions is well known and is especially useful for applying a primecoat to automobile bodies or other metal fabrications. In such a process, the item to be coated is immersed in the electrophoretic coating composition, and the coating is deposited onto the desired item by the application of electric current.
Titanium dioxide pigment is often used in electrophoretic coating compositions. However, commercially available grades of such pigment often can cause instability of such compositions and defects in the cured paint film. Accordingly, the need exists for a titanium dioxide pigment which does not have the foregoing undesirable properties.
The following references are disclosed as being of interest to the subject matter of this invention:
U.S. Pat. No. 3,635,810 is directed to electrocoating with desulfated pigments. This patent discloses that impurities which detrimentally affect the electrocoatability of the pigment composition are removed by treating the pigment composition with a water-soluble barium (cationic) compound. The specific impurity which is removed is a sulfate (an anion).
U.S. Pat. No. 3,591,478 is directed to an electrocoating process. The quality of electrocoated film is affected by the level of anionic contaminants in the coating bath.
U.S. Pat. Nos. 3,362,899 and 3,575,909 (related patents) disclose that it is advantageous that water-soluble salts in the pigment, such as sodium or potassium salts, be absent from the bath or present in very low amounts. Neither patent discloses how the water-soluble salts are removed nor do they suggest any acceptable levels.